1. Field of The Invention
The present invention relates generally to a lens barrel of an inner-focusing type. Specifically, the present invention relates to such lens barrel commonly installed in a video camera.
2. Description of the Background Art
Generally, an inner-focusing type of lens barrel in which groups of element lenses, such as compensator lenses or master lenses, are axially arranged to be microscopically moved along the optical axis of the lens barrel for focusing images is installed in a common video camera. The lens group is held by a movable lens frame installed in the lens barrel so as to axially move with the lens along the optical axis. FIG. 1 shows an example of such movable lens frame. A movable lens frame 100 having an approximately annular configuration holding a lens group 101, e.g., a variator lens group having a magnification varying mechanism for zooming images, in the inner circumference of the lens frame, moves with the lens 101 to the direction along the optical axis C of the lens barrel. A sleeve bearing 102 having an approximately H-shaped transverse section is integrally projected from the outer circumference of the lens frame 100 toward the upward direction thereof, and concurrently, extended along the optical axis C. The sleeve bearing 102 has a pair of tapped holes 102a and 102b respectively in a front and a back portion thereof. A lead screw axis 103 rotatably driven by a motor, not shown in the figure, screws through both tapped holes 102a and 102b. A flat spring 104 formed of elastic material is fixed on the side surface of the outer circumference of the lens frame 100 at a base member 104a thereof to convert radial rotation of the lead screw axis 103 to axial movement along the optical axis C. The flat spring 104 extends from a base member 104a toward the direction of the lead screw axis 103, and a flexible end 104b is extended from the spring 104 toward the lead screw axis 103 so as to press to gear the side portion thereof. Thus, side pressure P is applied from the flat spring 104 toward the screw axis 103 to control mechanical positioning hysteresis of the screw axis 103 during driving. A guiding slit 100b is formed at the lowest portion of the outside circumference of the lens frame 100 opposite the side from which the sleeve bearing 102 is projected. A guide axis 105 is located to slidably contact with the guiding slit 100b. The movable lens frame 100 is reciprocated by the reversible rotation of the lead screw axis 103 forwardly and backwardly along the axial direction of the optical axis C, concurrently, being guided by the guide axis 105. Thus, the lens 101 held by the lens frame 100 is reciprocated forwardly and backwardly along the axial direction of the optical axis C.
The flat spring 104 presses the lead screw axis 103 of the movable lens frame 100 by the side pressure P generated. In order to accomplish precise positioning of the lens frame 100 on the lead screw axis 103, the spring 104 must be easily deflected toward the direction of a Y-axis, and concurrently, must be hard to deflect toward the directions of an X-axis and a Z-axis, as shown in FIG. 1, i.e., the spring 104 must be formed so as to have a low spring constant with respect to displacement toward the Y-axis direction and a high spring constant with respect to displacement toward the X-axis and Z-axis directions.
However, because the flat spring 104 has a cantilever construction regarding with the Y-axis and Z-axis directions, it is difficult to control the spring coefficient to be low only toward the Y-axis direction. This causes precise positioning of the movable lens frame 100 to be difficult.